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Continuous furnaces are used for a variety of applications, such as the manufacture of electronic components. These furnaces often have a set of thermal or heating chambers within each of which the temperature and composition of the atmosphere are controlled. Product is advanced sequentially through each chamber at a determined rate to achieve a desired thermal and atmosphere profile.
Product may be advanced through continuous furnaces in various manners, for example, in one type of continuous furnace, the product sits on a metal mesh belt which pulls the product through the furnace. In another type, a continuous pusher furnace, the product is placed on plates or carriers or boats that are pushed into the entrance of the furnace. Each subsequent plate pushes the plate in front of it. A line of contacting plates is advanced by pushing on the rearmost plate in the line.
Often, it is desirable to operate two chambers within a continuous furnace at different atmospheres that must be kept separated. Typically, the chambers are spaced by tunnels or vestibules. Often doors at the entrance and exit of the chambers are provided to retain the atmosphere within the chamber. These doors, however, are costly and complex. To close the door in a continuous pusher furnace, product carriers in a contacting line must be separated, for example, by pushing the carrier at the head of the line at 90xc2x0 to move it off the line of travel and into a purge chamber or furnace section. A door is then closed behind the isolated carrier and the chamber purged. The carrier may than be advanced to the next chamber by another pusher along a line offset from the first line. This procedure must be repeated for each carrier. This requires additional furnace length, cost, and multiple pushers.
In the present invention, a continuous furnace incorporates a traveling gas barrier to create a barrier to open gas travel between the furnace chambers. During operation of the furnace, gas flows from one heating chamber, an upstream chamber, to an adjacent heating chamber, a downstream chamber. At the same time, gas may try to diffuse from the downstream heating chamber toward the upstream heating chamber, against the gas flow. The magnitude of the diffusion velocity could be greater than the magnitude of the gas flow velocity, in which case the composition of the atmosphere in the upstream chamber could be altered as the diffusing gas enters the upstream chamber. In the present invention, diffusion of gas from the downstream chamber into the upstream chamber is prevented by a gas barrier that travels with product through the furnace. The gas barrier ensures sufficient downstream gas velocity to overcome diffusion.
More particularly, the continuous pusher furnace has at least one heating chamber and typically a plurality of heating chambers. Vestibules interconnect the heating chambers. Entrance and exit vestibules are also typically provided. Gas containment from the process chambers to the outside through the entrance and exit vestibules operates in the same manner as chamber-to-chamber separation.
Each product carrier assembly comprises a pusher plate disposed to receive product thereon and a gas barrier extending upwardly from the pusher plate. The gas barrier has a perimeter sized and configured to fit within the vestibule with a clearance gap between the perimeter and the vestibule walls that increases the gas flow velocity through the vestibule sufficiently to overcome the gas diffusion velocity through the vestibule in a direction opposite to the gas flow. The traveling gas barrier of the present invention thus prevents diffusion of gas into the upstream chamber. The traveling gas barrier allows the furnace heating chambers to be aligned along a single line, thereby minimizing the size of the furnace. The need for complex doors and multiple pushers is eliminated, and product may be moved through the furnace more rapidly and efficiently.
In an alternative embodiment, one or more exhaust outlets are additionally provided in the vestibule or chambers to exhaust gas from both the upstream chamber and the downstream chamber out of the furnace. The length of the vestibule is selected to allow sufficient opportunity for the gas to be exhausted through the exhaust outlets.